2 Answers
2

you need more IP addresses to do things that can not be done in one IP address. Examples are FTP, or HTTPS (as most web servers require a separate IP address for every distinct SSL end point for technical reasons. Besides that there is no drawback and hosting multiple sites on one IP is required per IP distribution guidelines. Try getting one IP per site and they (the IP registry) will laugh about you for years to come for making such a request.

Besides that, the amount of sites you can host depends on the sites. How much memory they need, how much processing power they need, how much space they need - all that will run out at one point and then you need a larger system. THat simple. The number can go even below 1 - many sites use a cluster of servers to acutally publish them as one server is not fast enough.

Your answer was wrong there, it is wrong here, too. Wildcard or SAN/UCC certs are pretty often not the solution. Especially not in a general term. SNI is not available in most configurations, whether you call them modern or not. It is not supported by IIS and by many (linux) hosts, too. Ask them about TLS and you get empty looks ;) Sorry, reality sometimes sucks. It is a nice technical solution, though, once it actually IS feasible.
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TomTomJun 15 '10 at 13:56

You can host any number of sites under a single IP address unless you also intend to provide other services or SSL-based web services. SSL in the most common configuration requires a unique IP address per certificate. It is possible to share a single certificate across any number of hostnames, but this means you'd have to get them all in place first.

Some things are expected to use a single TCP or UDP port and so to use the same port with more than one name (and keep them distinct) you may need to keep them on different addresses. A common example is FTP.

Most FTP servers let you have virtual users, so you can open up one ftp server, and redirect any number of users to different root directories (so they can't see each other either). Also, you can do multiple SSL sites per IP address, but you have to use wildcard certificates, and they'll have to share a 2nd level domain name.
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JasonJun 15 '10 at 12:33